# Definition for singly-linked list.
class ListNode(object):
    def __init__(self, x):
        self.val = x
        self.next = None

class Solution(object):
    def deleteDuplicates(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        if not head: return None
        last = ListNode(None)
        cur = head
        while cur:
            if cur.val==last.val:
                last.next = cur.next
                cur = cur.next
            else:
                last = cur
                cur = cur.next
        return head